Dogs, Cats And Climate Change: What's Your Pet's Carbon Pawprint?

Dogs and cats are responsible for a quarter of the greenhouse gas emissions caused by animal agriculture, according a new study out Wednesday, which adds up to a whopping 64 million tons of carbon-dioxide equivalent emitted in the production of their food. But scientists remain divided about the role our pets play in global warming.

When researchers set out to assess the impact of personal actions on climate change, they expected dog ownership to be a large source of greenhouse-gas emissions.

Dogs tend to eat meat, after all, and meat production is a major contributor of greenhouse gas emissions. By one estimate, it's even the largest. And dogs tend to eat highly processed meat, so their carbon pawprint grows as their food is manufactured, and again as it is transported.

But Seth Wynes of Sweden's Lund University and Kimberly A Nicholas of the University of British Columbia did not find conclusive evidence that dog ownership contributes substantially to climate change, relative to other lifestyle choices. And some side effects of dog ownership may mitigate dogs' impact, they said, like the tendency of dogs to encourage low-carbon activities like walking and discourage high-carbon activities like airline flights.

"Further research in this area would be beneficial before making environmental recommendations about dog ownership to the public," they write in the journal Environmental Research Letters. "Still, we would suggest with some confidence that a smaller dog is likely to have a smaller carbon footprint than a larger dog."

Wynes and Nicholas compared three breeds to assess their relative impact: the Jack Russell terrier, Labrador retriever and St. Bernard. They assumed most of the dog's carbon pawprint would come from food, with only a negligible contribution from other sources like toys and trips to the vet.

They estimated the energy requirements of each breed, with guidance from the American Kennel Club, and the mass of food each breed needs to consume.

Then they just needed the carbon intensity of dog food, and that's when things got complicated. They could find only two peer-reviewed studies that offered a lifecycle assessment of the impact of dog food, and the two studies—one from Arizona State University and one from Australia—offered conflicting results.

Using data from the Australia study, they calculated the Jack Russell causes the emissions of about 2o kilograms of carbon dioxide per year, 60 kg for the Labrador, and 90 kg for the St. Bernard.

But using the ASU study, they calculated the Jack Russell emits 600 kg, the Labrador 1.6 metric tons and the St. Bernard 2.3 tons.

Graphic by J Emrys McMahon

This chart shows the disparate results found by two studies of the carbon footprint of dogfood.

"We found only two studies measuring the carbon footprint of dog food, and the studies gave very different estimates for the size of that impact," Wynes told me via email. "We would welcome further research on this topic as we found there was insufficient information available to draw firm conclusions."

The study that came out Wednesday, from Gregory S. Okin at UCLA, is too recent for consideration in the Wynes & Nicholas survey. It equivocates less about the impact of pets.

"As pet ownership increases in some developing countries, especially China, and trends continue in pet food toward higher content and quality of meat, globally, pet ownership will compound the environmental impacts of human dietary choices," Okin writes in PLOS ONE. "Reducing the rate of dog and cat ownership, perhaps in favor of other pets that offer similar health and emotional benefits, would considerably reduce these impacts. Simultaneous industry-wide efforts to reduce overfeeding, reduce waste, and find alternative sources of protein will also reduce these impacts."

Pet owners have heard this news before.

A reader alerted me to a 2009 book written by sustainable living specialists in New Zealand who assert that a medium-sized dog has an ecological pawprint twice that of a typical SUV. (The ecological footprint is a different measure than a carbon footprint: it measures the amount of land necessary to generate the dog's food). That claim made its own headlines when it came out. Wynes had seen it too, but it didn't meet the standard for inclusion in his study.

"I'm aware of the book you're referring to," Wynes said, "but unfortunately their calculations are not peer-reviewed, which is the gold standard for trustworthy research among scientists. This is one of the reasons we did not list dog ownership as a potential high-impact action."

Nonetheless, Wynes is confident that dogs who eat less food have smaller carbon pawprints than dogs who eat more food, giving small breeds an ecological advantage.

"We would suspect that cats, by extension, would have even smaller carbon footprints than most dogs."